1. Field of the Invention
The invention relates to a printing device and, in particular, a printer having a frame, an abutment retained therein for supporting a recording medium, a print head which is mounted on the frame so aa to be movable relative to the abutment in the direction of the print lines, an actuating drive for setting the print head distance relative to the abutment, and a pressing element which is fixed to the frame, presses the recording medium onto the abutment and can be deflected substantially perpendicular to the surface of the abutment in accordance with the thickness of the recording medium.
2. Description of the Related Art
U.S. Pat. No. 4,676,675 discloses an apparatus for adjusting the portion of a print head in a matrix printer in which a sensor intended for resting on the surface of the recording medium is arranged on the print head carrier. The print head carrier can be adjusted with the aid of an eccentric adjusting mechanism toward and away from the abutment. To determine the thickness of the recording medium, the latter is first drawn into the printing position. The print head carrier is then moved into a reference position which is located furthest away from the abutment and is determined by means of a light barrier. The distance of the sensor surface from the abutment in this reference position is stipulated and known. The print head carrier is subsequently moved toward the abutment until the sensor rests with a particular contact pressure on the surface of the recording medium which has in the meantime been drawn in, the path covered up to this point being measured by means of a counter. Subsequently, this measured route is subtracted from the previously known reference route in the arithmetic unit. The difference corresponds to the thickness of the recording medium. On the basis of the value thus determined for the thickness of the recording medium, the print head carrier is then moved away from the abutment again until the desired distance between the print head and the surface of the recording medium has been set.
The method described above is relatively cumbersome and complex. A further disadvantage in this solution is that the calculation of the thickness and thus also the setting of the head distance can only take place when the recording medium is already in the printing position. This leads to a considerable loss of time. Furthermore, the distance between the abutment and the sensor must not be greater than the distance between the abutment and the print head. On the other hand, however, it must not be substantially smaller either since it would otherwise not be possible to set the distance between the surface of the recording medium and the print head to a desired value. This requires not only a relatively precise adjustment of the sensor relative to the print head, but can also cause difficulties when paper of a different thickness is drawn in.
In order to eliminate the disadvantages described above, it has already been proposed in an apparatus of the type known from U.S. Pat. No. 4,676,675 to arrange a sensor ahead of the printing position--seen in the drawing--in direction of the recording medium--on a part which is fixed to the printer frame relative to the abutment in such a way that, when the recording medium has not yet been drawn in, it rests on the abutment and can be deflected or deformed by the recording medium being drawn in, the deflection or deformation of the sensor being converted into a proportional electrical parameter. The sensor proposed in this solution comprises movable parts which become soiled due to the unavoidably occurring paper dust and can thus become jammed. Furthermore, these mechanical parts would have to be adjusted.
The document German Published Application 26 08 301 C 2 furthermore discloses an apparatus for controlling the distance of a print head from a recording medium, in which a sensor in the form of a two-armed lever is mounted so as to be pivotable on the print head carrier or the print head carriage in such a way that it can rest with the one free end of the lever on the recording medium in the region of the respective printing point. The sensor is formed from a thin metal sheet which is pressed by a spring with a very small contact pressure against the recording medium such that the sensor can slide over the paper virtually without resistance. At its contact point, the metal sheet has a cutout, through which the needles of the print head can pass. At its lever end facing away from the contact point, this sensor bears a magnet which interacts with a Hall probe arranged on the print head carrier. The print head carrier or the print head slide is moved perpendicular to the abutment via an electronic control circuit until a reference voltage, corresponding to a predetermined distance of the print head from the recording medium, occurs on the Hall probe. This solution has considerable disadvantages in practical operation. On the one hand, the sensor must rest only very lightly on the surface of the recording medium as it could otherwise impede the movement of the print head and damage the recording medium during the movement of the print head. On the other hand, however, it is precisely this small contact pressure which leads to the sensor reacting to any unevenness in a recording medium. This is particularly noticeable in multi-layered recording media, for example sets of forms or savings books. However, momentary deflections of the sensor lead to a permanent readjustment of the print head, as a result of which the printing performance of the printing device is considerably impaired. Additionally, the setting of the correct distance can only take place when the recording medium has already reached the nozzle of the print head, i.e. is in its recording position. Since the sensor is situated in the direct vicinity of the print head nozzle, past which the ink ribbon also runs, there is the risk that the sensor will be damaged when the ink ribbon is changed. This risk is all the greater as the sensor has to be very thin and of light-weight construction for reasons of space and function. Furthermore, since the sensor is pressed onto the recording medium by a spring and the pressing force is to be only very small, a slight change in the pressing force already leads to the sensor being more or less pressed into the recording medium, which occurs particular with multi-layered recording media. Consequently, it is necessary for the pressing spring to be adjusted very precisely. In practical experience, this is extremely difficult. In the case of weak springs, in particular, a slight bending of the spring is generally enough to change its pressing force considerably. It is therefore exceptionally difficult to adjust a sensor of this type and to maintain the adjustment during operation.